1. Field of the Invention
The present invention relates to a wireless LAN (Local Area Network) system and a communication method therefor. More specifically, the invention relates to a wireless LAN system supporting multiple classes of service requiring high priority of traffic and including a VoIP (Voice over Internet Protocol) apparatus, a moving picture communication apparatus and the like for transporting multimedia information. Further, the invention relates to a method for wireless communication in a wireless LAN system for transmitting multimedia information whose delivery should be ensured within the constraints of QoS (Quality of Service) parameters such as transmission delay and delay fluctuation.
2. Description of the Background Art
IEEE (Institute of Electrical and Electronic and Engineers) 802.11 standard defines wireless LAN operations. The wireless LAN connection supports ad-hoc and infrastructure modes. The ad-hoc mode is defined such that mobile stations (STAs) on a wireless LAN are allowed to communicate directly with each other and there are no administrative stations fixed in the network. The infrastructure mode is designed such that a wireless LAN is formed by a base station or node, which is commonly referred to as an access point (AP), through which communications are established with the mobile stations to function as an administrative station in the network.
In the wireless LAN, a frame sequence is basically designed such that an ACK (ACKnowledge) frame is transmitted in response to a unicast frame transmitted. However, when a frame such as a multicast or broadcast frame is to be transmitted to a plurality of destination stations, an ACK response is not required.
The IEEE 802.11 standard defining the infrastructure mode provides two access methods. One of the access methods is DCF (Distributed Coordination Function) using a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol. The other is PCF (Point Coordination Function) using the CSMA/CA protocol. The PCF function cooperatively control media through access points.
Each of the mobile stations under the control of DCF function is required to determine whether wireless media are free before transmitting a frame. As a result of the determination, if the mobile station determines that the wireless media are being used by another station, i.e., it is busy, the mobile station will abort the right of transmission on the wireless media to the other station until the other station completes transmission of a frame. Eventually, after the transmission of a frame completes and a back-off time has elapsed, the mobile station can transmit a frame which the station wishes to transmit. The back-off time is specific to that mobile station, and defined by a specified time interval DIFS (Distributed Inter-Frame Space) and a random number.
The master-slave relationship as formed by the PCF function is designed to include a base station, access point, or master station, with mobile stations serving as slaves is not applied to the DCF function. The DCF function is designed such that every station, including an access point, can equally access the media except for the case of the station transmitting a beacon, for example
Further, under the control of PCF function, mobile stations are allowed to transmit a frame only when they receive a polling signal from an access point. In a PCF network, the right to transmit on the media is centrally managed and controlled by an access point, and therefore the stations before transmission are neither required to determine whether or not the wireless media are free, nor to wait until the back-off time is over.
In the CSMA/CA protocol defined as a primary protocol of the DCF function, there are two methods for allowing a wireless station to sense a carrier to determine whether or not the wireless media are available for transmission. One method is a physical carrier sensing in which a radio frequency (RF) module functioning as a physical layer is used to sense a carrier wave. The other method is a virtual carrier sensing which uses a network allocation vector (NAV) set in various kinds of frames transmitted on the wireless media. On a MAC (Media Access Control) layer, both methods are combined to effectively decrease the probability of collisions caused by a number of stations wishing to transmit on the media. The physical carrier sensing is implemented by the RF module, and the virtual carrier sensing is implemented by using results obtained from the MAC layer to determine the presence or absence of a carrier on the wireless media at intervals of up to one microsecond.
The latter method, i.e., the virtual carrier sensing uses the NAV vector. The NAV vector is set in all mobile stations, and is a kind of counter that starts decrementing at a constant rate to “0” immediately after a value of the NAV vector is set. The value of the NAV vector is representative of the remaining duration of a period of time during which a mobile station is scheduled to currently transmit or receive a frame on the wireless media. When the NAV vector decrements to “0”, the wireless media are free, i.e., in the idle state thereof.
A mobile station of interest wishing to transmit or receive a frame sets in advance in the duration/ID field of the MAC header of a frame to be transmitted a duration scheduled by the station to occupy the wireless media after completing the transmission of a frame currently being transferred. All mobile stations other than the mobile station of interest wishing to transmit or receive a frame read the appropriate field of a frame transmitted on the wireless media, and set the value of the field in the respective NAV vectors at the time of completing the transmission of the frame. Note that if the value of the NAV vector of a mobile station at the moment of setting the NAV vector is greater than the value read from the duration/ID field of the frame being transferred, the station does not set the value read therefrom.
Next, how to control the NAV vector will be generally described. All mobile stations other than a mobile station of interest wishing to transmit or receive a frame set the value specified in a duration/ID field of a data frame in their own NAV timers at the time of completing the transmission of the data frame. The NAV timer decrements at a constant rate, and reaches the value “0” at the time of completing the transmission of an ACK frame. The duration in which the NAV timer has its count not equal to zero is the duration during which the media are busy.
A time interval between a data frame and an ACK frame is called SIFS (Short Inter-Frame Space), which is specific to the PHY layer. When only physical carrier sensing is used, the media are determined to be free for this time interval. However, when virtual carrier sensing is used, the media are determined to be continuously in the busy state thereof until the end of an ACK frame. In contrast, when transmitting data first, the NAV vector is not yet set but physical carrier sensing function is used to cause stations to recognize the busy state of the media. In this way, on the MAC layer, physical carrier sensing and virtual carrier sensing are combined to detect whether or not the media are busy, thereby significantly reducing the probability of mobile stations colliding. If the duration field of an ACK frame counts down to “0”, the media are rendered free at the end of the ACK frame.
Any mobile station under the control of DCF function can exchange an RTS/CTS (Request To Send/Clear To Send) frame prior to transmission of a desired frame. This allows a mobile station to first exchange an RTS/CTS frame of short length with an actual destination mobile station and to check information on the destination mobile station and a transmission path up to the destination station. However, for example, when a data frame that the mobile station wishes to transmit is essentially short, it is less efficient to use the RTS/CTS frame, and instead the MAC layer uses a predetermined RTS/CTS threshold parameter to determine whether or not an RTS/CTS frame should be used. Basically, an RTS/CTS frame is not applied to a frame of which the length does not exceed the RTS/CTS threshold but only to a frame whose length exceeds the threshold. To an RTS/CTS frame, the NAV vector is also controlled as with an ordinary data frame and the like.
The RTS/CTS frame sequence and the NAV control will be described. In an RTS/CTS frame sequence, for the time intervals or spacing between RTS and CTS frames, between CTS and data frames and between data and ACK frames, use can be made of the short inter-frame space (SIFS) for transmission, which is the shortest interval among all the frame intervals regulated in the standard. Accordingly, if a mobile station has successfully transmitted an RTS/CTS frame, the mobile station is able to have the priority right to transmit on the wireless media to all other mobile stations trying to transmit a frame after the DIFS spacing, which is a time interval longer than the SIFS spacing. Accordingly, it is more likely for the mobile station to successfully transmit data frame/ACK frame following the RTS/CTS frame.
The value in the duration/ID field of a frame indicates a duration from completion of the transmission of the frame to completion of the transmission of an ACK frame, thus meaning that the duration/ID field of the last ACK frame contains the value of “0”.
For example, as a result of virtual/physical carrier sensing, the wireless media are busy and then rendered free to allow a required time interval, typically the DIFS period, to elapse. Immediately thereafter, if plural mobile stations waiting for transmitting data try to transmit the data, then the media access by those stations collide against each other, causing the probability of collision to increase.
Thence, there are methods for avoiding collision, among which the IEEE 802.11 standard defines a collision back-off procedure. How the back-off procedure is performed will be described briefly. If three mobile stations #1, #2 and #3 are waiting to transmit data and use virtual/physical carrier sensing to determine that the wireless media are busy or involved in the DIFS period thereof beginning immediately after the busy period terminated, the mobile stations are getting ready to assign the right to transmit on the wireless media. After a frame has been transmitted and the DIFS period elapses, the media are rendered free i.e., in the idle state thereof. At this moment, if all mobile stations attempt to simultaneously transmit a frame, the timing of access is rushed into a short period of time, and therefore the probability of collision increases.
In order to reduce the probability of collision, each mobile station must refrain from transmitting on the media for the duration of a randomly chosen period, i.e., a back-off time, following the DIFS period. This allows reduction in the probability of collision between multiple media accesses.
In addition to the DCF access method previously described, the IEEE 802.11 standard for wireless media access method specifications optionally defines a PCF access method, which is usable on infrastructure network configurations. The PCF function is designed so that a master station, called a point coordinator (PC) and ordinarily serving as an access point, centrally manages the rights of individual mobile stations to transmit on the wireless media. Accordingly, unlike the DCF function, there is no conflict between individual mobile stations wishing to transmit to get the right to transmit.
Conventionally, wireless LAN systems were primarily directed to dealing with traffic of data streams. However, the recent development and deployment of multimedia technologies such as Voice over IP (VoIP) increases an ongoing need for providing wireless LAN systems with the capability to support multimedia traffic, such as audio and video, to the same extent as data traffic conventionally done.
The multimedia traffic is characterized by its periodicity and undurability to delay. Multimedia traffic periodically generated by a transmitting source may be subject to delay exceeding a threshold, fluctuation in transmission delay and/or unacceptable loss of information over a transmission network. In such a case, a destination receiver may reproduce audio and/or video data, etc., with the quality thereof degraded to a level unable to evaluate.
The aforementioned DCF access method under IEEE 802.11 is basically designed for sending an unpredictable asynchronous burst of data in an efficient manner. Accordingly, it is expected with high probability that data transmission delay varies significantly depending on contention over a network. It is considered difficult for multimedia traffic characterized by periodic and synchronous information to enjoy the advantages attained by reducing transmission delay and variance in transmission delay to maintain better QoS (Quality of Service). Further, the PCF access method under IEEE 802.11 standard is optimized for periodic and synchronous multimedia traffic. However, the PCF mode is essentially an option based on the DCF function and currently is not widely introduced. Therefore, the PCF access method does not offer a practical solution to the problems faced by wireless LAN systems.
Thus, for stations functioning under the DCF access method widely available, there is a need to reconfigure only access points so as to control the QoS of wireless LANs compatible to multimedia traffic.
Some specific proposals will be presented below. The first proposal has to satisfy a condition that, in a method for facilitating control of QoS by equipment in a wireless LAN base station in an IEEE 802.11 wireless LAN system, the base station is connected to a network to form infrastructure mode where mobile stations communicate under the control of the DCF function and is receiving a first frame from a mobile station, and further has a high-priority frame for multimedia application to transmit after receipt of the first frame. The proposal further has to satisfy an additional condition that the base station specifies in a duration/ID field of a signal responsive to the first frame a duration necessary to transmit the high-priority frame for multimedia application, and transmits the response signal.
The second proposal is that a base station sets a duration necessary to transmit a high-priority frame for multimedia application in the duration/ID field of a first frame and transmits the first frame. In the third proposal, a base station sets in a duration/ID field of a signal responsive to a first frame a duration predicted to be required for receiving a high-priority frame for multimedia application, transmits the response signal, and thereafter executes a specific sequence of causing a mobile station of interest to reset its own NAV timer. In the fourth proposal, a base station sets in a duration/ID field of a first frame a duration predicted and required for receiving a high-priority frame for multimedia application, transmits the first frame, and thereafter executes a specific sequence of causing a mobile station of interest to reset its own NAV timer.
According to these proposals, when the high-priority frame for multimedia application is transmitted and received between the base station and the mobile station, a value of non-zero is set in the duration/ID field and the specific sequence is used to cause the mobile station of interest to reset its own NAV timer, thereby remarkably increasing the probability for a base station or mobile stations to have the right to transmit on the media and thus improving the QoS.
As described above, in an application focusing on providing better QoS connection over a wireless LAN, a base station provided with a capability to control and maximize the QoS on a wireless LAN should be installed in a wireless LAN system.
Examples of how the base station controls the QoS and is installed in a wireless LAN system will be described in detail below.
U.S. patent application publication Nos. US 2002/0159418 A1, US 2002/0131371 A1 and US 2002/0163928 A1 disclose a method for assigning priority levels to wireless stations communicating in a network and providing better Quality of Service (QoS) connection to a device that conforms to the IEEE 802.11 standard. This method includes grouping stations into a polling list set, selecting a number of the grouped stations for inclusion in a polling list subset, in which preference is given to high-priority QoS stations in the polling list subset, and polling the high-priority stations during a contention-free period.
Another U.S. patent application publication, No. US 2003/0185186A1, discloses a wireless LAN system intended to improve the QoS on a transmission line. Each base station in a wireless LAN system comprises a header analyzing circuit determining a priority of data transmitted from a wireless terminal based on a priority queue table, a buffer storing data based on the determined priority, a transmission control circuit transmitting the data stored in the buffer based on the priority to a destination, and a control circuit updating the priority queue table on the basis of priority information transmitted from a host apparatus. The control circuit references the information contained in the priority queue table of the base stations, assigns priorities to the data to be transmitted to each of the wireless terminals, and transmits information indicative of the priorities assigned to the data to each base station.
Still another U.S. patent application publication, No. US 2003/0186724 A1, discloses a base station in a wireless LAN system intended to improve QoS on a transmission line. A base station comprises a memory for storing a priority data table indicative of priorities of data set for every application, a control circuit for determining the priority of received data on the basis of the table to output data having an assigned priority value to a first buffer and data having no priority to a second buffer, a beacon signal generation circuit for transmitting a beacon signal at a constant interval, and a transmission control circuit for transmitting, as indicated by the beacon interval, the data stored in the first buffer to a destination at a constant interval and after having transmitted the data stored in the first buffer, transmitting the data stored in the second buffer to a destination. A base station assigns a priority specifically to an application. The priority is assigned with a finer level than that in the conventional method. The probability that base stations concurrently receive the data having the same priority is lower, and only the same data is stored in the first buffer. The same data stored in the first buffer is transmitted at a constant interval and then the wireless terminal receives the data at a constant interval.
However, when a base station already exists in a wireless LAN network but is not provided with the aforementioned capability to control and maximize the QoS, an application utilizing the wireless LAN network and focusing on providing a better QoS connection requires the base station to be configured to support the QoS required by the application. When the base station is configured to support the QoS thus required by the application, there would be a problematic situation in which the base station will not make use of existing equipment assets in the future.